When scientists study extreme heat, they often rely on daily minimum, maximum or average temperatures. Heatwaves are also commonly measured by how many days temperatures remain above a certain threshold. But looking at heat one day at a time can hide what happens during individual hours.
A new study led by Weilin Liao and Xuanzong Zhang at Sun Yat-sen University examined global heat extremes hour by hour. Using temperature records from 1981 to 2010 to establish hourly thresholds, the researchers found that extreme heat is becoming more frequent and that hot hours are increasing faster than hot days, as per an Ars Technica report.
The analysis suggests that by the end of the century, an average hot day could contain several more hours of extreme heat, depending on future greenhouse gas emissions.
Researchers measured heat one hour at a time
Instead of treating each day as a single temperature measurement, the researchers assessed every hour separately.
For example, temperatures recorded at 8 a.m. were compared only with other 8 a.m. temperatures. An hourly heat extreme was defined as a temperature above the 90th percentile for that particular hour during the 1981–2010 reference period.
This approach can reveal periods of extreme heat that daily measurements may overlook. A day might not qualify as a "hot day" overall but could still contain several hours of unusually high temperatures.
Over the past four decades, people around the world have experienced an average of about 270 hours of hourly heat extremes each summer. That figure has been increasing by about 62 hours every decade.
Hot days could become much longer
The researchers then used climate model projections under two future greenhouse gas emissions scenarios. One scenario represents medium emissions and about 3°C of global warming by 2100. The other represents high emissions and about 5°C of warming.
Compared with the past couple of decades, the total number of hourly heat extremes between 2070 and 2099 is projected to triple under the medium-emissions scenario. Under the high-emissions scenario, the number could increase 4.5 times.
The study also found that every additional 0.5°C of global average warming is associated with about 200 more extreme-heat hours per summer, as per the Ars Technica report.
An average hot day could gain more than four hours of extreme heat
The hourly analysis also showed how the experience of a hot day could change as the planet warms. At present, an average hot day contains about 16 hours of extreme heat.
Under the medium-emissions scenario, that could increase to 19 hours, and under the high-emissions scenario, it could reach 21 hours. That means an average hot day could contain five more hours of extreme heat under the high-emissions scenario than it does today, leaving fewer hours when people experience some relief from the heat.
Hot hours can appear even when a day is not classified as hot
The hourly approach revealed another part of the problem. Some days that do not meet the definition of a hot day can still contain several extreme-heat hours, particularly at night. Those shorter periods can occur before or after hot days, extending the amount of time people are exposed to heat.
Daily measures can miss these episodes because they focus on whether temperatures cross a threshold over the course of an entire day.
Hot nights are becoming a bigger part of heat exposure
The researchers found that extreme-heat hours are increasing more at night than during the day, as per the Ars Technica report. That matters because simply counting hot days would not capture these additional periods of nighttime heat.
The trend is also strongest in the tropics, even though overall warming is greater at higher latitudes. The researchers attribute this to changes in temperature variability in the tropics, which make extreme temperatures more frequent.
Low-income countries face the strongest increase
The study also examined how extreme-heat exposure is changing across countries with different income levels. Low-income countries experience the strongest increase in hot hours.
The researchers also compared lifetime exposure between people born in 1981 and those born in 2023, as per the Ars Technica report.
At the global average, people born in 2023 are projected to experience about 1.4 to 1.8 times as many extreme-heat hours over their lifetimes, depending on the future greenhouse gas emissions scenario.
Hourly data reveals what daily averages can miss
The overall increase in extreme heat is already well established. What this analysis adds is a closer look at how that heat is distributed throughout the day.
Extreme-heat hours are increasing faster than the number of hot days. Hot nights are becoming more common, while shorter periods of extreme heat can occur on days that would not otherwise be classified as hot.
The researchers argue that examining heat at an hourly scale will become increasingly important for understanding health risks, planning climate adaptation and developing early warning systems.
As temperatures rise, the problem may not simply be that there are more hot days. The hot days themselves could contain longer periods of extreme heat, while additional hot hours appear before, after and even on days that daily measures would otherwise overlook.
